Nanochannel glass replica membranes

ABSTRACT

The present invention is a process for making a nanochannel glass (NCG)  rica, having the steps of: coating a face of an etched NCG with a replica material (with or without an intervening buffer layer), where the etched NCG face has a plurality of channels arranged in a desired pattern, to form a replica coating on the NCG conforming to the pattern; and removing the replica coating from the etched NCG. The present invention is also the replica made by this process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods for making structures thatmimic the patterns in nanochannel glasses.

2. Description of the Related Art

In complex structures, the relative placement of features is animportant concern, along with the size and resolution of the features.It is a continuing goal in miniaturization efforts (e.g., electronics)to make structures, including freestanding (i.e., structures notdisposed on a supporting substrate) structures, with nanoscale features(e.g., ≧10⁸, 10⁹, 10¹⁰, 10¹¹, etc. features/cm²). It is particularlydesired to make these structures of various materials that can be usedas masks for a variety of parallel processing techniques. Masks madefrom etched NCG have been used in these processes. However, for featuresizes smaller than 1 μm, the aspect ratio of the etched NCG increasesrapidly. For instance, the lowest aspect ratio obtained to date in anetched NCG mask for a 1 μm feature size has been about 4:1(thickness/feature width). Desirable masks will have controllable aspectratios.

As used herein, nanochannel glass (NCG) refers to a composite ofdifferent glasses, where these glasses are arranged in selectednanoscale patterns. Typically, NCG will be a composite of two glasses,referred to as the "matrix glass" and the "etchable glass". Uponexposure to some agent(s) or condition(s), the etchable glass willdissolve or otherwise be removed from the matrix glass, and the matrixglass will be unaffected, or at least minimally affected by theseconditions. Such conditions include, but are not limited to, exposure toa solvent such as an acid, a base, reactive ions, or water. "Etched NCG"refers to the NCG that has been at least partially (and optionallycompletely) developed by exposure to an agent or condition that willdifferentially remove glass from the NCG, and "channel" refers to thevoids created by this removal of glass, regardless of the geometry ofthese voids. It is this property of having the different glassesarranged in selected patterns, with high accuracy (ca. 0.5% of channelsize), high precision (high repeatability), and small, controllableminimum feature sizes (ca. 10 nm or less), that distinguishes NCG fromother composite glasses. Likewise, it is the property of having voidsarranged in selected patterns that distinguishes etched NCG from otherporous glasses (such as Vycor™).

Currently, conventional high resolution lithographic masks with smallfeature sizes are made by serial e-beam lithography, or (more recently)by AFM/STM patterning. These require a large number of processing steps,as well as a pixel-by-pixel exposure of the mask.

In particular, it is desired to provide masks for making structures withregular arrays of features, such as quantum electronic devices, magneticstorage media, and nonlinear opto-electronic devices.

For these types of devices with nanoscale features, a continuingobstacle is the inability to combine fine nanoscale features, highpacking densities (e.g., ≧10⁹ features/cm²), and large expanses (e.g.,an entire wafer). For instance, in the aforementioned serial e-beamlithography, as the e-beam writing process progresses, there is atendency for the beam to deviate (or lose global registration) from itsintended or programmed location, due to positioning instabilities. Thiseffect can be somewhat mitigated by the use of fiducials, but theproblem is inherent to e-beam lithography. In addition to the problem ofglobal registration, long write times are required to write a largenumber of features. Thus, even if one was willing to go to the (probablyprohibitive) expense of trying to expose a large wafer (e.g., a 6"wafer) with a large number of nanoscale features, one would likely findthe technical problems insurmountable at present.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide structureswith nanoscale features (voids), which may or may not be freestanding.

It is a further object of this invention to provide an inexpensivemethod for making such structures.

It is a further object of this invention to make these structures withcontrolled aspect ratios, including low aspect ratios.

It is a further object of the invention to make these structures withlarger overall areas, higher packing densities, and greater numbers offeatures than are currently available through other methods, such ase-beam lithography.

These and additional objects of the invention are accomplished by thestructures and processes hereinafter described. The present invention isa process for making a nanochannel glass (NCG) replica, having the stepsof: coating a face of an etched NCG with a replica material (with orwithout an intervening buffer layer), where the etched NCG face has aplurality of channels arranged in a desired pattern, to form a replicacoating on the NCG conforming to the pattern; and removing the replicacoating from the etched NCG. The present invention is also the replicamade by this process.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention will be obtained readilyby reference to the following Description of the Preferred Embodimentsand the accompanying drawings in which like numerals in differentfigures represent the same structures or elements, wherein:

FIG. 1 shows the sequence of steps to the invention in block diagramform.

FIG. 1a shows the sequence of steps to the invention in block diagramform, where this process includes using an optional buffer layer,

FIG. 2 shows a replica material partially endcapped over a buffer layeraccording to the invention.

FIG. 3 shows a replica material fully endcapped over a buffer layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Nanochannel glasses with preselected patterns will be made followingtechniques previously taught by the inventors. The following areincorporated by reference herein, in their entireties, for all purposes:

(a) U.S. Pat. No. 5,332,681, issued Jul. 26, 1994 to Tonucci et al.[Tonucci '681];

(b) U.S. Pat. No. 5,264,722, issued Nov. 23, 1993 to Tonucci et al.[Tonucci '722];

(c) U.S. Pat. No. 5,306,661, issued Apr. 26, 1994 to Tonucci et al.[Tonucci '661];

(d) Tonucci et al., "Nanochannel Array Glass", Science 258 783-85 (Oct.30, 1992);

(e) Pearson et al., "Nanochannel Glass Replica Membranes", Science 27068-70 (Oct. 6, 1995);

(f) U.S. patent application Ser. No. 08/725,211, filed on date evenherewith for "PARALLEL CONTACT PATTERNING USING NANOCHANNEL GLASS", anddesignated as Navy Case No. 76,713.

Referring to FIG. 1, the invention is preferably practiced as follows. Asample of etched NCG 10 is prepared so that a face 12 of the etched NCG10 has a pattern 14 of channels 16 corresponding to the pattern desiredfor the replica. Typically, the pattern-bearing face of the NCG ispolished to a high degree of flatness, using available glass polishingtechniques.

Alternatively, for a curved (concave or convex) replica, thepattern-bearing face of the NCG is polished to fit (convex or concave)this curved replica. As used herein, "convex" and "concave" indicate thesign of the curvature, and are not limited to particular geometries.Typically, the polishing step is performed before the channels in theNCG are developed (i.e., before the etchable glass is removed).

Very large NCGs with very large area patterns are made by iterating thebasic process for making NCGs, as described in the references supra.

A replica material 20 is applied to the pattern-bearing face 12 of theetched NCG 10, so that this applied material carries the same pattern asthe pattern-bearing face of the etched NCG (i.e., the etched NCG faceacts as a master for the applied material). A variety of techniques maybe used to deposit the materials to be applied to the etched NCG. Theseinclude line-of-sight (LOS) deposition techniques such as thermalevaporation, electron beam evaporation, molecular beam epitaxy, andlaser ablation, nearly LOS techniques such as sputtering at lowpressure, and (in some instances) non-LOS techniques such as chemicalvapor deposition (CVD) and sputtering at high pressure. Practitionerswill choose between these techniques for depositing metals (hardferromagnetic, soft ferromagnetic, antiferromagnetic,non-ferromagnetic), semimetals, semiconductors, insulators,superconductors, organics (including polymers), etc.

The distinguishing factor between LOS and non-LOS deposition techniquesin this context relates to the trajectories of the deposited atoms. InLOS techniques, the depositions usually take place at pressures of 10⁻⁵torr and below, and the deposited atoms travel undeflected away from thesource to the substrate in straight lines. In nearly LOS techniques,such as sputtering at low pressures, the deposited atoms suffercollisions with gas molecules and other atoms between the source and thesubstrate, slightly reducing the directionality of the deposited atomscompared to LOS techniques.

In non-LOS techniques, such as sputtering at high pressures and chemicalvapor deposition, the deposited atoms arrive at the substrate withminimal directionality, coating all outer surfaces essentiallyuniformly.

Aspect ratios may be controlled by controlling the deposition of thereplica material, and the size of the voids in the etched NCG. For manypatterning applications aspect ratios of less than 1.0, e.g., 0.5, 0.1,0.05, etc., are desired. For other applications higher aspect ratios maybe desired, e.g., 5, 10, 50, 100, and greater.

It has been demonstrated that a wide range of metal replica materialsincluding platinum, palladium, gold, molybdenum, and tungsten can bedeposited on an etched NCG face and subsequently be removed intact toform an NCG replica. Since Mo and W are considered to be the mostdifficult metals to make into an NCG replica according to the presentinvention, and since making NCG replicas with each of these metals hasbeen demonstrated, the invention is considered to be practicable withany of the transition metals, as well as other metals. In principal, anysolid material which can be deposited by methods such as those describedabove is amenable to the inventions, particularly if a suitable bufferlayer is available.

Ordered, disordered, and amorphous metal alloys, composites, andmultilayers are all available as replica materials in the presentinvention. Alloys may be made by co-deposition, or by deposition from analloy source. Composites may be made by annealing alloy replicas afterremoval from the etched NCG. Multilayer replicas may be made bysequential deposition. Such multilayers will have a variety ofapplications. For example, metals (hard ferromagnetic, softferromagnetic, antiferromagnetic, non-ferromagnetic), semimetals,semiconductors, insulators, superconductors, organics (includingpolymers), etc., could be sequentially deposited to make complex layeredstructures. Likewise, a sequence of layers could be deposited to provideoptimized surface and bulk effects. As an example, a thin Ti layer, athicker Pt layer, and another thin Ti layer have been deposited toprovide a structure with good chemical resistance (from the Ti), andgood mechanical properties, e.g., ductility (from the Pt). It may bedesirable to control the properties of the interface between adjacentmetal layers by controlling the transition from one layer to the next,e.g., by forming a selected thin alloy interlayer between adjacentlayers.

Optionally, as shown in FIG. 1a, a buffer material 18 may be depositedin a layer between the pattern-bearing face 12 of the etched NCG 10 andthe applied material 20. Such buffer layers 18 typically will be used tofacilitate the removal of the etched NCG 10 from the applied material20. Aluminum is the preferred buffer layer for most replica metals,except for aluminum itself. Copper is the preferred buffer layer foraluminum replicas. Metal buffer layers can be removed easily with acidsolutions or base solutions.

Suitable buffer layer materials include semiconductors, insulators,ceramics, metals such as aluminum, alloys that are soluble in acids orbases, polymers that are soluble in organic solvents, and various watersoluble materials such as sodium metaphosphate, other salts, andhygroscopic glasses. Sodium metaphosphate, for example, is a highlyeffective buffer layer. Besides being water soluble, it has a number ofadvantages, including that it can be evaporated, and that it istypically amorphous when evaporated (and therefore there is not theproblem of grain growth interfering with the sharpness of the bufferlayer). Other water soluble buffer layer materials are sodium chloride,and other salts.

Buffer layers may deposited by any suitable LOS method, and also bynearly LOS and non-LOS methods, such as chemical vapor deposition (CVD)and wet chemistry.

As seen in FIG. 2, partial endcapping (or pinch-off) of the replicamaterial 20 over the optional buffer layer 18 and the etched NCG 10, canadvantageously provide replicas with smaller apertures than the channels16 in the etched NCG 10. However, as seen in FIG. 3, care should betaken so that this pinch-off effect does not result in the replicamaterial 20 completely endcapping the buffer layer 18, so that thebuffer will be inaccessible to solvents and therefore will not provideliftoff.

After the replica material is coated onto the etched NCG (with orwithout the intervening buffer layer), the applied material is removedfrom the etched NCG, with the pattern thereon intact, thereby formingthe NCG replica. Depending on the particular applied material, and themanner in which it is applied, the method of removing the material tothe pattern-bearing face of the NCG will vary. For example, if a solublebuffer layer is interposed between the applied material and the etchedNCG, the coated etched NCG may be contacted with a solvent fordissolving the buffer layer, thereby allowing the intact replica to beremoved from the etched NCG.

Alternatively, the coated etched NCG is placed in contact with a solventthat is selected to (a) dissolve the etched NCG, and (b) not damage theapplied material.

Alternatively, replica materials and NCGs may have differentialcoefficients of thermal expansion, which may be exploited (by heating orcooling) to remove the replica from the NCG.

NCG replica membranes according to the present invention may befreestanding, or may be supported on a variety of substrates, including(but not limited to) semiconductor (e.g., silicon), insulator,superconductor, organic (including polymer), ceramic, and metal (e.g.,copper, gold, aluminum, iron, and various metal alloys) substrates.

Using the processes described herein, a person of ordinary skill in theart can make NCG replica membranes with exceptional minimum featuresize, packing density, total number of features, and patterned areas.Moreover, exceptional combinations of these features likewise can beobtained. For example, replica membranes with patterned areas that haveat least 10⁴ features, typically at least 10⁷ features, more typicallyat least 10⁸ features, preferably at least 10⁹ features, more preferablyat least 10¹⁰ features, still more preferably at least 10¹¹ features, ormost preferably at least 10¹² features therein are attainable by thepresent invention. It is contemplated that replica membranes with 10¹⁵features can be made by the present invention. Likewise, replicamembranes with patterned areas that are at least 1" across, typically atleast 2" across, more typically at least 4" across, preferably at least6" across, more preferably at least 8" across, or most preferably atleast 12" across are attainable by the present invention. Also, replicamembranes with patterned areas that have local packing densities of atleast 10⁸ features/cm², typically at least 10⁹ features/cm², moretypically at least 10¹⁰ features/cm², preferably at least 10¹¹features/cm², more preferably at least 10¹² features/cm², or mostpreferably at least 10¹³ features/cm² are attainable by the presentinvention. Also, replica membranes with patterned areas that have globalpacking densities of at least 10³ features/cm², typically at least 10⁵features/cm², more typically at least 10⁷ features/cm², preferably atleast 10⁹ features/cm², more preferably at least 10¹¹ features/cm², ormost preferably at least 10¹³ features/cm² are attainable by the presentinvention. It is contemplated that replica membranes with packingdensities of 10¹³ features/cm² can be made by the present invention.Furthermore, replica membranes with patterned areas that have minimumfeature sizes of submicron size, below 200 nm, typically below 100 nm,preferably below 50 nm, more preferably below 20 nm, or most preferablybelow about 10 nm are attainable by the present invention.

Having described the invention, the following examples are given toillustrate specific applications of the invention, including the bestmode now known to perform the invention. These specific examples are notintended to limit the scope of the invention described in thisapplication.

EXAMPLE 1 Fabrication of a Replica Membrane

As an illustration of this process, a hexagonally patterned platinumreplica membrane was fabricated to be 75 nm in thickness with voids 40nm in diameter. This membrane was prepared by mechanically polishing thesurface of an NCG wafer 0.9 mm in diameter using progressively smallerdiamond grits to a final grit size of 0.25 μm. After polishing, thewafer was briefly acid etched, exposing the voids to a depth ofapproximately 1.5 μm, and was then coated with 90 nm of aluminumfollowed by 75 nm of platinum using planar magnetron sputtering in argongas. The sample was then slowly immersed in a solution of 3 g sodiumhydroxide in 15 ml deionized water at a temperature of 60° C., quicklydissolving the aluminum and releasing the platinum replica membrane. Themembrane was rinsed in deionized water and picked up with a 1000 meshcopper grid for SEM characterization. An SEM micrograph of this membraneis shown in FIG. 3 of Pearson et al., supra. The membrane has an aspectratio slightly less than two and contains a packing density greater than3×10⁹ voids/cm².

The replica membrane shown in FIG. 3 of Pearson et al. was prepared froman etched NCG wafer with voids 130 nm in diameter which are considerablylarger than the voids present in the membrane itself. This reduction inthe void diameter is a result of the sputtering process, as noted supra.

EXAMPLE 2 Use of a Replica Membrane in Substrate Patterning

To illustrate the use of these membranes in substrate patterning, aplatinum replica membrane was prepared to be 110 nm in thickness withvoids 115 nm in diameter, for use as a deposition mask. The membrane wasprepared from an etched NCG wafer 1.65 mm in diameter with voids 240 nmin diameter. After polishing and etching, the NCG wafer was sputtercoated with 100 nm of aluminum followed by 110 nm of platinum. Themembrane was released from the etched NCG wafer and rinsed, and was thenplaced on a silicon (110) substrate and allowed to dry. 30 nm ofplatinum was sputtered onto the silicon (110) substrate, through thereplica membrane. The replica membrane was then removed with adhesivetape, leaving an array of platinum dots on the surface of the siliconsubstrate. An SEM micrograph of the silicon substrate following thepatterned platinum deposition and removal of the membrane is shown inFIG. 4 of Pearson et al., supra. Similar experiments have also beencarried out at larger feature sizes, resulting in the patterneddeposition of tungsten, molybdenum, gold, and nickel on varioussubstrates. It has been found that these membranes maintain theirpattern integrity at the elevated temperatures which may be encounteredin deposition processes. This stability is a benefit compared toconventional resists used in lithographic processes, which can sufferpattern degradation under the deposition of refractory materials.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A process for making a replica of an etchednanochannel glass (NCG), comprising the steps:coating a face of anetched NCG with a replica material, wherein said etched NCG face has aplurality of channels arranged in a pattern therein, to form a replicacoating on said etched NCG conforming to said pattern; and removing saidreplica coating from said etched NCG.
 2. The process of claim 1, whereinsaid process comprises the additional step of coating said etched NCGface with a buffer material, prior to said step of coating said face ofsaid etched NCG with said replica material, wherein said step of coatingsaid buffer-coated face of said etched NCG with said replica materialcomprises depositing said replica material onto said buffer-coated faceunder conditions selected to prevent complete capping of saidbuffer-coated face, and wherein said step of removing said replicacoating from said etched NCG comprises removing said buffer material,therein removing said replica coating from said etched NCG.
 3. Theprocess of claim 2, wherein said buffer material is selected from thegroup consisting of metals, semimetals, ceramics, organics, glasses, andsalts.
 4. The process of claim 2, wherein said buffer material isselected from the group consisting of aluminum, copper, sodium chloride,hygroscopic glass, and sodium metaphosphate.
 5. The process of claim 2,wherein said buffer material is a polymer.
 6. The process of claim 2,wherein said buffer material is amorphous.
 7. The process of claim 1,wherein said step of removing said replica coating from said etched NCGcomprises dissolving said etched NCG.
 8. The process of claim 1, whereinsaid replica coating is amorphous.
 9. The process of claim 1, whereinsaid replica coating is crystalline.
 10. The process of claim 1, whereinsaid etched NCG and said replica coating have different coefficients ofthermal expansion, and wherein said step of removing said replicacoating from said etched NCG comprises bringing said etched NCG and saidreplica coating to a temperature where said replica coating delaminatesfrom said etched NCG.